The present invention relates to the field of electronic circuits, and more particularly, to DC-DC converters and associated methods.
Typically, DC-DC converters use current flow information to provide value added functions and features. For example, limiting the current during an overload is commonly implemented as a safety feature. Such a current limit feature would use a signal proportional to output current limiting level. A resistor inserted between the output and the load could generate the desired temperature-independent feedback signal. However, the resistance of this sensor is the subject of a trade-off between power dissipation and signal amplitude. Typically, the signal level at current limit is approximately 0.1 volt, to be well above the noise floor. The sensing resistor""s power dissipation is proportional to the load current at the limit level. At high current levels, the power dissipation can be excessive.
Eliminating the sensing resistor improves the DC-DC converter""s efficiency. Instead of an additional resistive element, current flow is measured using the intrinsic elements within the power converter components. For example, U.S. Pat. No. 5,982,160 to Walters et al. and entitled xe2x80x9cDC-to-DC converter with inductor current sensing and related methodsxe2x80x9d teaches that the current flow information in an inductor can be reconstructed as a voltage across a resistor-capacitor network. This method uses the intrinsic resistance of the inductor""s winding as the current sensing element.
Another method to eliminate the current sensing resistor measures the voltage dropped across the nearly constant, on-state resistance of one of the switching MOSFETs in the converter. The method samples the voltage drop during the conduction of interval of the MOSFET to reconstruct the current flow information. Both of these methods make use of the fundamental power converter components as current sensing elements and they avoid using a dissipative element in the power path.
A problem with the use of intrinsic elements is the variation of their conductivity with temperature. Copper (the most popular material used in fabrication of electronic conductors and magnetic windings) has a resistivity temperature of coefficient of 0.0039/xc2x0 C. Similarly, the channel region of a MOSFET transistor also exhibits a positive temperature coefficient whose variation with temperature can often be approximated using a straight line. The slope varies with the technology used, interpolating to a temperature coefficient of 0.0041/xc2x0 C. to 0.0065/xc2x0 C. (considering select technologies from several contemporary MOSFET manufacturers).
As current travels through these circuit elements, it generates heat proportional to its magnitude. In most systems, and specific to electronic power converters, the controlling circuit is never situated too far away from these power elements. The heat generated in the power elements (also used for sensing purposes) heats up the surrounding areas, the controlling circuit and ultimately, the local ambient temperature, as well.
In view of the foregoing background, it is therefore an object of the invention to reduce or eliminate the thermal effects present in intrinsic circuit elements used to sense various parameters in electronic circuits.
This and other objects, features and advantages in accordance with the present invention are provided by an integrated control circuit for an electronic circuit, such as a DC-to-DC converter, using current sense information developed across an intrinsic circuit element of the electronic circuit. The integrated control circuit includes a control unit to control the electronic circuit, and a temperature sensor to sense a temperature rise induced by the intrinsic circuit element of the electronic circuit and output a temperature error signal. A variable gain amplifier is connected to the temperature sensor to generate an amplified temperature error signal, and an adder is connected to the variable gain amplifier to combine the amplified temperature error signal and a feedback signal from the intrinsic circuit element of the converter, and to provide a temperature-corrected feedback signal to the control unit.
The intrinsic circuit element is preferably an output inductor or a power switch of a DC-to-DC converter. The power switch would typically be a power field effect transistor (FET) and the current sense information is measured as the voltage dropped across the on-state resistance of the power FET. Preferably, the adder eliminates an induced temperature error from the feedback signal thereby creating a temperature-independent feedback signal. Also, the gain of the variable gain amplifier may be adjustable via an external adjustment circuit.
Alternatively, the temperature sensor may have a positive temperature coefficient and sense a temperature rise induced by the intrinsic circuit element of the electronic circuit. Here, an amplifier receives a feedback signal from the intrinsic circuit element of the electronic circuit and the temperature error signal from the temperature sensor to generate a temperature-corrected feedback signal to the control unit. The amplifier preferably eliminates an induced temperature error from the feedback signal to generate a temperature-independent feedback signal. Also, the temperature sensor may receive an adjustment signal from an external adjustment circuit.
Another aspect of the present invention relates to method of regulating a DC-to-DC converter as described above. The method includes sensing current in an intrinsic circuit element, such as the output inductor or the power switch, of the converter. The current feedback loop circuit is operated in cooperation with the pulse width modulation circuit to control the at least one power switch in response to the current sensor. The method further includes generating a temperature error signal by sensing a temperature rise induced by the intrinsic circuit element of the converter, and providing a temperature-corrected feedback signal to the pulse width modulation circuit by combining the temperature error signal and a feedback signal from the current feedback loop circuit.
Also, the temperature error signal may be amplified with a variable gain amplifier connected to the temperature sensor. Providing the temperature-corrected feedback signal preferably includes eliminating an induced temperature error from the feedback signal to provide a temperature-independent feedback signal.
Alternatively, the method may include generating a temperature error signal by sensing a temperature rise induced by the intrinsic circuit element of the converter with a temperature sensor having a positive temperature coefficient, and generating a temperature-corrected feedback signal to the pulse width modulation circuit based upon a feedback signal from the current feedback loop circuit and the temperature error signal from the temperature sensor.